Unless we are captured by alien slavers and dragged there, we are not going to be around when (if) human starships reach the seven rocky dwarf planets orbiting tightly around TRAPPIST-1. Why wait for that – when we can just make stuff up. Come on – NASA wants you to!
If you want a fictional setting for novels or role-playing games that has space travel, but you do not want the Hand-wave physics of FTL, or the sitting-in-a-can-trying-not -to-lose-your-mind-while being-baked-by-radiation reality of realistic space-flight, the recent trove of planets hugging a tiny star called TRAPPIST-1 is your answer.
TRAPPIST-1 (named for the Chilean telescope array they used to study it) is a M dwarf star, in this case about 8% of the mass of our own sun, located only 40 light years away towards the constellation Aquarius. In the fall of 2016, scientists used 500 hours of Spitzer time (an orbiting infrared telescope) to search for exoplanets and found a horde.
No less than seven were found, all in or near Trappy’s theoretical habitable zone; the “Goldilocks Zone” where liquid water might be present. These would be very close orbits – well within the orbit of Mercury. The closest [b] has an orbital period of 1.5 days, and the farthest [h] has an orbital period of about 20 days. They are fairly close to Earth sized, only one is more than 40% removed by estimated mass. (d is pretty small at .41 – but that still makes it larger than Mars). They are all most likely tidally locked, meaning one side always faces the Trappy while the other side always faces space.
This is about all that’s known about them. Studies are planned to try and determine atmospheric composition and the like, but that’s going to take years. Let’s make stuff up.
Three of these worlds (e, f and g) are in the Good Green Zone of habitation, meaning the possibility of Earth like temperatures. However, those models kinda assume the worlsd spins on an axis relative to the sun – which is not likely the case here. But, the tidal locks may mean that the dark sides of b, c and d may be cool enough, and the light side of h may be warm enough.
All of that, though, depends upon what atmospheres they have which could be anywhere from zippo like Mercury to crushing acidic greenhouse of doom like Venus. Let’s speculate to limit some variables:
B is really close, and probably really irradiated. H is actually almost twice as far out as g, and h is the one they don’t have good mass estimates on. Let’s say both of those are barren. Let’s also go with the odds and say that tiny d doesn’t have enough atmosphere to hold a robust biosystem. That still leaves you four worlds.
Inhabitants of one of these planets could get to their nearest neighbor with not much more effort than it took us to get to the moon. We’re talking weeks – not years (as would be the case going to Mars) and that’s a far more manageable technical feat, but in terms of supplies and radiation. So whether you’re imagining human colonizers or native inhabitants or both – they could readily get to the point of back and forth between worlds.
So you could multiple sets of competing ecosystems: not only dark-side and light side, but native vs nearby world and native vs distant colonizer transplants. That’s a niches to hide weird monsters in.
If I am commanded by editorial Gods to come up with a setting based on this system, I’m actually doing a steampunk setting – except replace coal with wind – which on a tidally locked planet is both fierce and constant. Civilization might cluster arund the meridian belt seperating the light and dark side. Shadows are basically permanent; the Sun barely moves along the one horizon. On the other horizon, only clouds will hide the stars.
The entire meridian would be circumnavigated by train, of course.
That’s just the one that’s most Earth-like. That’s the one humans landed on and finally, after much struggle, seized control of. The natives then fled to the nearby planets, where they huddle together with former rivals in coalition against these new invaders for forty light years away.
Why steampunk? Humans aren’t supposed to be there. Anything as advanced as radio would give us away to Powerful Enforcers.
M dwarfs and similar stars make up 75% of known stars – they are the default, at least in our neighborhood of the galaxy. If TRAPPIST-1 is typical, we may be surrounded by more rocky neighbors than we imagine.
Similar situation with a gas giant in hab zone: